The mechanisms by which animals and humans process the chemical sensory information that permit them to smell and taste are not understood. This problem will be investigated using the leech, a powerful model system in which, to a much greater extent than in higher animals, the cells of the nervous system can be studied directly and individually. The elements of the leech chemosensory system(s)-including the peripheral chemoreceptors, the nerves leading from them to the central nervous system, and the neurons in the central nervous system that are involved in chemosensory processing-will be identified and characterized using the methods of extracellular and intracellular electrophysiological recording, light and electron microscopy, and metabolic mapping of active neurons with 3H-2-deoxyglucose. Induced in the characterization of these elements will be a description of the way they interact with each other to allow discrimination of different chemical stimuli. This work has two long range goals: 1) to discover how chemical sensory information is processed by the nervous systems of animals, including humans, and 2) to further characterize the nervous system of the leech, a valuable model system in neurobiological research, and one in which new methods and principles have repeatedly been developed and later applied to the study of higher animals.